Oxidation of hydrocarbons and oxygen carrier therefor



Nov. 12, 1957 E, c. HUGHES ETAL OXIDATION OF HYDROCARBONS AND OXYGENCARRIER THEREFOR Filed Jan. 27, 1954 fiwomzoumo 05 5 3 0 rl C D n 3 O 0%6 e. P U D 5 Ht 0 M -5 P P F um o 7 5 0 0 NH 5 0 US m 6 AG ll. F ..0 3 O2 \I F a w O m O I 2 m I\ l I I I 0 0 0 O O m 8 6 2 TIME HOURS) I YFIGZ.

TEMP. F.')

FIGS.

/o CV03 ON SiO swim K S C RGE www O E THT M N S EC. )0 VTA n WT A EL RR0 \C|. E H VAY T EH l TIME(HOURS) 2,8 13,1 14 Patented Nov. 12, 1957OATION OF HYDROCARBONS AND OXYGEN C 2! 1 m R THEREFOR ApplicationJanuary 27, 1954, Serial No. 406,446 3 Claims. (Cl. 260-451) The presentinvention relates to a process for the reforming of hydrocarbons bypartial oxidation.

The petroleum industry has devised many processes for the reforming ofhydrocarbons, one type of which is classed as oxidative reforming. Ingeneral, oxidative reforming processes are characterized by thereforming of a hydrocarbon in the presence of oxygen or a substancecapable of liberating oxygen. In one species of oxidative reforming, thehydrocarbon is contacted with an oxygen-containing gas, such as air, atan elevated temperature. In another species of oxidative reforming, thehydrocarbon is contacted with a metallic oxide in the absence of anyadded quantities of air or an oxygencontaining gas under conditionswhich liberate oxygen from the metallic oxide. In the latter species ofoxidative reforming, the main reaction which occurs is one between theoxygen of the metal oxide and the hydrocarbon although slight catalyticeffects are also present. The oxidation of a hydrocarbon using a solidoxygen carrier offers several advantages over oxidation in the presenceof an oxygen-containing gas since the problem of separating the productfrom the reactants when both include fixed gases is eliminated and, inaddition the reaction occurs at a lower temperature, thereby eliminatingside reactions such as cracking.

The present invention is concerned with the latter species of oxidativereforming, i. e., the reforming of a hydrocarbon in the absence of anadded oxygen-containing gas but in the presence of a metal oxide whichwill liberate oxygen under the conditions of the reaction.

The latter species of oxidative reforming is illustrated in U. S.Patents Nos. 1,836,325 and 1,836,326 to J. H. James. In the Jamesprocess, the metal oxide used as an oxygen carrier may be an oxide oftitanium, vanadium, chromium, manganese, zirconium, niobium, molybdenum,tantalum, tungsten or uranium. The oxide can be used as a finely dividedmaterial in a fluidized type of process or can be enclosed betweenscreens or similar supports in a fixed bed type of process.

When chromium trioxide is utilized in a process of the type described byJames, the liberation of oxygen is known to'proceed according to thefollowing equation:

"iCrO; 130 203 2 Of the two valence states represented in the formula,the trivalent oxide of chromium (CrzOs) is thermodynamically stable attemperatures below about 1000 F. The hexavalent chromium trioxidedecomposes quantitatively to trivalent chromic oxide at about 1020 F.but the reverse reaction does not take place below about 2190 F. Fromthe equation, it is apparent that chromium trioxide in the ordinary caseis not readily adaptable as an oxygen carrier for the production ofoxygen because excessively high temperatures are required forregeneration from the reduced form.

It is a primary object of the present invention to pro- The above objectand others are achieved by a process 1 utilizing chromium trioxidedeposited on silica gel as an oxygen carrier in the oxidative reformingprocess. In essence the process of the invention comprises contacting ahydrocarbon in the vapor phase at an elevated temperature in the absenceof an added oxygen-containing gas with an oxygen carrier composed ofchromium trioxide deposited on silica gel to reduce the chromiumtrioxide and reform the hydrocarbon, recovering the reformedhydrocarbon, and regenerating the reduced oxide of chromium by heatingin the presence of an oxygen-containing gas such as air.

In accordance with the present invention, it has been found that whenchromium trioxide is deposited on silica gel, the thermostability of thechromium trioxide is increased to such an extent that the chromium caneasily be reduced to an intermediate valence state less stable thantrivalent chromium. The meta-stable system that results from depositingchromium trioxide on silica gel can be represented by the followingequation:

GOO-800 F. GCrOa 2Cl203-C1'O3 30.

While the precise structure of the reduced oxide on the right hand sideof the equation is not known, the average valence of four, as indicatedby the formula, has been determined. From. the equation, it is apparentthat only relatively low temperatures which do not Widely differ arerequired in both the reduction and regeneration steps. The extent towhich the reaction will proceed in either direction is largely dependentupon the concentration of oxygen in the surrounding atmosphere. Thus, inaccordance with the general laws of chemistry, a high concentration ofoxygen will shift. the equilibrium toward the left in the above equationwhereas, when the concentration of oxygen is low, the equilibrium willshift toward the right.

The successful operation of the process of this invention is subject tocriticalities in the temperatures employed for reduction, i. e.oxidation of the hydrocarbon, and regeneration and in the amount ofchromium trioxide relative to the silica gel in the oxygen carrier. Thetemperature employed in the reduction step is. perhaps the leastcritical but it should be within the approximate range of 300 to 950 F.Temperatures below this range do not result in significant improvementof the hydrocarbon and temperatures above the range lead to anundesirable amount of decomposition of the chromium trioxide to thedifficulty oxidizable C1'2O3, thus interfering with the ease ofregeneration.

The temperature of regeneration should be within the approximate rangeof 600 to 800 F. The percent regeneration at temperatures below 600 F.is too low for satisfactory operation, and above 800 F. irreversibledecomposition of the intermediate oxides to CrgOs results at anincreasing rate. Optimum regeneration is obtained within the range of700 to 750 F. The time of regeneration invention may be produced inaccordance with procedures well known in the art. The preferredprocedure involves the impregnation of silica gel with a solution of achromium compound followed by evaporation of the solvent to yield a drysolid and thereafter converting the'chromium compound to the trioxide byheating in air. Alternatively, the silica gel may be impregnateddirectly with a solution of chromium trioxide in which case conversionto the oxide form is unnecessary. i

This invention and the advantages thereof will be described inconnection with the attached drawings wherein:

Figure 1 is a graph comparing the percent decomposition into CrzOa ofunsupported chromium trioxide and silica gel-supported chromiumtrioxide, when heated at atmospheric pressure; V

Figure 2 is a graph on which is plotted percent regeneration vs. thepercentage by weight'of chromium trioxide in the oxygen carrier; 7 i V iFigure 3 is a graph on which is plotted percent regeneration vs.temperature employed in the regeneration at atmospheric pressure of asilica gel-supported chromium oxide obtained by reduction of chromiumtrioxide supported on silica gel;

Figure 4 is a graph on which is plotted percent regeneration vs. time inhours for the regeneration of a silica gel-supported chromium oxide atatmospheric pressure.

The data for the decomposition curve on unsupported chromium trioxidefor Figure 1 was obtained from the literature, Nargund and Watson]. Ind.Inst. Sci., 9A, 149 (1926). The data for the decomposition curve onsilica gel-supported chromium trioxide for Figure 1 was determinedexperimentally. It can be observed from Figure 1 that unsupportedchromium trioxide readily decomposes to ClzOs even at temperatures aslow as 660 F. On the other hand, silica gel-supported chromium trioxideis relatively stable as regards decomposition to CraOs. At 1050 F. thedecomposition of the supported oxide is less than the decomposition ofthe unsupported oxide at 660 F., and at temperatures lower than 1050 F.the decomposition of the supported oxide is almost negligible.

The data for Figure 2 was obtained by blowing air at a temperature of800 F. for one hour through beds of reduced silica gel-supportedchromium oxide containing various weight percentages of chromium oxide.It can be observed from Figure 2 that the percent regeneration begins todrop sharply as the concentration exceeds 20% and that about 25% byweight is the upper limit for reasonably eflicient operation. Lowerconcentrations of oxide are not harmful but, for practical reasons, itis desirable to employ as high a percentage of oxide as possible withoutexceeding 25 by weight. A lower limit of about 10% by weight of theoxide is suggested.

.The curve of Figure 3 was obtained by blowing air at varioustemperatures through beds of a silica gel-supported chromium oxideobtained by the reduction of silica gel-supported chromium trioxide. Theratio of chromium oxide to silica gel was approximately 1 to 9 and theregeneration was carried out in each instance for a period of one hour.From the graph it is apparent that maxi-. mum regenerating efliciency isobtained in the range of about 700 to 750 F. When the temperature ofregeneration exceeds 800 F. there occurs irreversible decomposition ofthe intermediate oxides to CrzOa at an increasing rate, and therefore800 F. represents a fairly critical upper limit for the regenerationtemperature. .As the temperature of regeneration falls .below 700 F.,the efficiency of regeneration drops sharply, and at 600 F. the percentregeneration is approximately 30%. Therefore, 600 represents theapproximate minimum temperature of regeneration which can be employed atatmospheric pressure to obtain reasonably satisfactory efficiency in theregeneration step. g

Figure 3 further reveals that a maximum regeneration of almost 85% canbe achieved by blowing heated air at atmospheric pressure through thereduced oxyen carrier,

4 Other tests have shown that the same approximate level of regenerationcan be achieved through several cycles and the maximum number of cyclesthrough which the oxygen carrier will remain effective appears to beextremely large.

The experiments on which th e curve of Figure 4 is based were performedby blowing air for different periods of time through beds of a silicagel-supported chromium oxide which was obtained by the reduction ofsilica gelsupported chromium trioxide in which the weight ratio ofsilica gel to chromium trioxide was 8 to 2. The temperatureofregeneration in each experiment was 750 F. From the curve of Figure 4 itis apparent that maximum efliciency of regeneration'is approached'aftera period of one hour and that littlebenefit is realized by regenerationtimes longer than one hour, although longer times are not harmful andmay be used if desired. The shortest regeneration period which can beemployed to give a practicable operation of the process at atmosphericpressure is in the neighborhood of ten minutes.

The manipulative steps of the process of the invention are conventional.Any of the various'types of reactors which are utilized in the vaporphase processing ofhydrocarbons can be employed. Either a fixed bedreactor or a fluid type catalytic reactor is suitable but sincefluidized reactions are generally advantageous, it is preferable tocarry out the process under fluidized conditions.

In order to further illustrate the process and its accompanyingadvantages, the following examples are given. Parts and percentages areby Weight unless otherwise specified.

Example 1 v V Forty-seven (47) parts of a commercial silica gel wasadded-to an aqueous solution of 3 parts by Weight of chromium trioxide(CrOa) in 50 parts of water. The resulting mixture was dried for severalhours in an oven at 290 F. to' yield a granular solid materialcontaining 6% of chromium trioxide.

The apparatus employed in the example resembled a conventional fluidizedbed catalytic reactor; It consisted essentially of a hopper to containthe solid oxygen carrier, a valve to control the flow of solid into thereaction zone, a reactor with conventional facilities for vaporizing andintroducing the hydrocarbon, a vapor solids separator, a solidsreceiver, a hydrocarbon recovery system and a regenerating vessel forthe oxygen carrier. Nitrogen pressure equivalent to 2 to 3 centimetersof mercury was maintained on the feed hopper to insure even 'flow of thesolid and to prevent hydrocarbon vapor from entering the feed hopper.

A quantity of the naphtha was oxidized in the reactor at a temperatureof 875 F. using a weight ratio of oxygen carrier to naphtha of 13.6:1.The spent oxygen carrier was then transferred to the regenerator andregenerated with hot air at a temperature of 750 F.

' The oxidized naphtha from the reactor was collected in a liquid statein the cooled receiver and the yield of liquid productwas found to equal83% of the naphtha charged. The liquid was analyzedfor Kattwinkel numberwhich was found to be 13, an increase of 2.5 over the original naphtha.Since the Kattwinkel number of a hydrocarbon is the measure of the sumof the aromatics, olefins and oxygenated compounds present, it isapparent that the process of the invention results in the formation of asignificant amount of oxidized products, olefinic and aromaticcompounds. As a result of these chemical changes in the charge stock,the octane properties are considerably improved.

Example 2 An oxygen carrier was prepared according to Example 1 exceptthat in the preparation 41.5 parts of commercial silica gel was added toan aqueous solution of 8.5 parts of chromium trioxide in 50 parts ofwater. The oxygen carrier prepared in this manner analyzed about 17.2%chromium trioxide.

The oxidation of the naphtha was carried out at 930 F. by a proceduresimilar to Example 1 employing a weight ratio of oxygen carrier tonaphtha of 53:1. The liquid product collected amounted to 83% of theoriginal naphtha charged to the reactor and the product was found tohave a Kattwinkel number of 13. Again a significant improvement in theoctane number of the naphtha was realized.

The oxygen carrier was regenerated air at a temperature of about 750 F.

It is intended to cover all changes and modifications in the examples ofthe invention, herein given for purposes of disclosure, which do notconstitute departure from the spirit andscope of the appended claims.

We claim:

1. In a process for the oxidative reforming of hydrocarbons, the stepsof heating to a reforming temperature within the range of 300 to 950 F.,materials consisting as before by hot essentially of a hydrocarbon andan oxygen carrier comprising a preformed silica gel impregnated withchromium trioxide, the amount of chromium trioxide not exceeding about25% by weight of the oxygen carrier, thereby reducing the chromiumtrioxide to a lower oxide down to but not including chromic oxide andreforming the hydrocarbon by the formation of oxidized products,olefins, and aromatics; recovering the reformed hydrocarbons; andreoxidizing the reduced chromium oxide to chromium trioxide by heatingin the presence of an oxygen-containing gas at a temperature of fromabout 600 to 800 F.

2. A process according to claim 1 in which the process is operatedcontinuously with alternate periods of reduction and reoxidizing.

3. An oxygen carrier consisting of chromium trioxide deposited on silicagel, the amount of chromium trioxide not exceeding about 25 by Weight ofthe oxygen carrier.

References Cited in the file of this patent UNITED STATES PATENTS1,836,325 James Dec. 15, 1931 2,351,793 Voorhees June 20, 1944 2,366,372Voorhees Jan. 2, 1945 2,371,087 Webb et al. Mar. 6, 1945 2,658,858 Langet a1. Nov. 10, 1953 2,718,535 McKinley et a1. Sept. 20, 1955

1. IN A PROCESS FOR THE OXIDATIVE REFORMING OF HYDROCARBONS, THE STEPSOF HEATING TO A REFORMING TEMPERATURE WITHIN THE RANGE OF 300 TO 950*F.,MATERIALS CONSISTING ESSENTIALLY OF A HYDROCARBON AND AN OXYGEN CARRIERCOMPRISING A PREFORMING SILICA GEL IMPREGNATED WITH CHROMIUM TRIOXIDE,THE AMOUNT OF CHROMIUM TRIOXIDE NOT EXCEEDING ABOUT 25% BY WEIGHT OF THEOXYGEN CARRIER, THEREBY REDUCING THE CHROMIUM TRIOXIDE TO A LOWER OXIDEDOWN TO BUT NOT INCLUDING CHROMIC OXIDE AND REFORMING THE HYDROCARBON BYTHE FORMATION OF OXIDIZED PRODUCTS OLEFINS, AND AROMATICS; RECOVERINGTHE REFORMED HYDROCARBONS; AND REOXIDIZING THE REDUCED CHROMIUM OXIDE TOCHROMIUM TRIOXIDE BY HEATING IN THE PRESENCE OF AN OXYGEN-CONTAINING GASAT A TEMPERATURE OF FROM ABOUT 600 TO 800*F.